1. Field of the Invention
The present invention relates to an ultrasonic blood flow imaging apparatus for acquiring blood flow information within a subject under examination utilizing the ultrasonic Doppler effect, and displaying the blood flow information as a two-dimensional image.
2. Description of the Related Art
Ultrasonic blood flow imaging apparatuses combine an ultrasonic Doppler method and a pulse-echo method to produce blood flow information and tomograph image (B mode) information by means of one ultrasonic probe, and simultaneously apply these pieces of information to a television monitor to visually display the superposed images of the blood flow profile and the tomograph image in colors on the monitor.
The above blood flow imaging is based on the following principle.
When being transmitted into a living subject within which blood flows, an ultrasonic beam is scattered by moving blood cells and subjected to the Doppler effect with the result that its center frequency fc is changed by fd. The frequency f of the ultrasonic echo undergone the Doppler effect becomes f=fc+fd. The frequencies fc and fd are are related as follows: EQU fd=2v.times.cos .theta./c.times.fc (1)
where v is the blood flow velocity, .theta. is an angle made by the ultrasonic beam and the blood vessel, and c is the sound velocity.
Accordingly, the blood flow velocity v can be found by detecting the Doppler shift frequency fd.
To measure the blood flow velocity by utilizing the Doppler effect as described above, ultrasonic pulses are repeatedly transmitted several times from an ultrasonic transducer into the living subject in a given direction. The ultrasonic pulse echo waves from the subject, which have been subjected to the Doppler effect, are received by the ultrasonic transducer, and then sequentially converted into echo signals. The echo signals are applied to a phase detector to detect Doppler shift signals. In this case, the Doppler shift signals are detected for, for example, 256 sample points along a steering direction of the ultrasonic pulses. The Doppler shift signals detected at the respective sample points are frequency-analyzed by a frequency analyzer, and then converted into a video signal by a digital scan converter (DSG) in order to display a blood flow profile image on the TV monitor.
The above operation is performed for each of a plurality of steering lines, so that a flow velocity profile corresponding to the steering lines is two-dimensionally displayed.
The flow velocity profile is conventionally displayed in gray. It is difficult, therefore, to discriminate between the flow velocity profile and the tomograph which are superposed and displayed on the TV monitor. Consequently, the velocity of the blood flow in a direction is subdivided into several velocity ranges, and a predetermined color is allocated to each velocity subdivision. The allocated colors are displayed according to the blood flow velocity, and the blood flow velocity is judged on the basis of the displayed color. With such a method, however, it is difficult to recognize a relationship between a specific color and a direction of the blood flow. To obviate this difficulty, a method has been devised which displays the direction of blood flow in red (for one direction) or in blue (for the opposite direction), and expresses the blood flow velocity through changes in the brightness (intensity) of red or blue. In the case of this method, however, it is very difficult to differentiate wide changes in blood flow velocity. This is because, even if the brightness of the red color is changed with a change in the blood flow velocity, this change of the brightness can hardly be perceived. Hence, it is difficult to determine the change of the flow velocity. Further, because of the correspondence of the flow velocity to the brightness of a color, a low flow velocity is indicated by a dark red or blue owing to a reduction in the brightness. Thus, the low flow velocity becomes difficult to find out.